1. Field of the Invention
The present invention generally relates to plasma-assisted deposition, in particular to a method for forming a dielectric film by plasma-assisted deposition using two-frequency combined pulsed RF power.
2. Description of the Related Art
As device miniaturization progresses, dielectric constants of interlayer insulation films which insulate intercalation need to be lower. Ideally, air-gap structures having a dielectric constant of one (k=1) are used. However, even with the air-gap structures, sidewalls of patterned recesses facing each other via an air gap need to be covered by an insulation film having a certain thickness. Films formed on sidewalls of patterned recesses are typically thinner than those formed on top surfaces of the recesses, but it is required that the films be stable and dense so as not to be apt to change properties with time and not to release gas to the air gaps. A SiO film is typically used as an insulation film formed on sidewalls of patterned recesses, but the SiO films formed on the sidewalls by conventional plasma-enhanced CVD are porous and have a lower density and higher wet etch rate than those of films formed on the top surfaces.
When the films formed on sidewalls are porous films having high degassing, the air-gap structures are subject to defects such as expansion, cracking, etc. during subsequent processes using high temperature such as film deposition processes and CMP processes. Thus, the films formed on sidewalls need to have properties equivalent to or better than those of films formed on the top surfaces.
When a film is formed on patterned recesses of a substrate by plasma CVD, it can be confirmed that a film formed on a top surface of the recesses is thick whereas a film formed on sidewalls of the recesses is thin. This tendency depends upon the size of line spaces and the depth of lines, and the smaller the line space, and the greater the aspect ratio of the line, the lower the thickness of a film formed on the sidewalls becomes relative to the thickness of a film formed on the top surface. Further, the properties of a film formed on the sidewalls and a film formed on the top surface are different; the film formed on the top surface is dense and has good quality due to ion bombardment generated by a plasma, whereas the film formed on the sidewalls is porous and has a low density because the sidewalls receive less ion bombardment.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made.